Negative electrode for secondary battery, and method for producing same

ABSTRACT

A negative electrode for a secondary battery, and a method for manufacturing the same, and more particularly, to a negative electrode for a secondary battery used for a negative electrode of a secondary battery, and a method for manufacturing the same. A negative electrode for a secondary battery may include a carbon-based active material; a conductive material; a silicon-based active material-polymer binder combination including a silicon-based active material, and a polymer binder for suppressing the expansion of the silicon-based active material bonded to a particle surface of the silicon-based active material; a thickener; and a water-based binder.

TECHNICAL FIELD

The present disclosure relates to a negative electrode for a secondarybattery, and a method for producing the same, and more particularly, toa negative electrode for a secondary battery used for a negativeelectrode of a secondary battery, and a method for producing the same.

BACKGROUND ART

Recently, interest in energy storage technology has increased. As theapplication fields are expanded to energy of mobile phones, camcorders,notebook PCs, and even to that of electric vehicles, efforts forresearch and development of electrochemical devices are more and morematerialized. In this respect, electrochemical devices are receiving themost attention, and in particular, the development of a rechargeablesecondary battery has become the center of attention. Recently, indeveloping such batteries, the research and development for the designof a new electrode and a new battery have been conducted in order toimprove a capacity density and specific energy.

Among secondary batteries currently being applied, a lithium secondarybattery has advantages over conventional batteries such as N-MH, Ni—Cd,and sulfuric acid-lead batteries which use an aqueous electrolyte inthat the operating voltage thereof is higher and the energy densitythereof is much greater.

In general, in a lithium secondary battery, materials capable ofintercalating and deintercalating, or alloying and dealloying lithiumions are used as a negative electrode and a positive electrode, and anorganic electrolyte or a polymer electrolyte is filled between thenegative electrode and the positive electrode to manufacture the lithiumsecondary battery. When lithium ions are intercalated and deintercalatedfrom the positive electrode and the negative electrode, electricalenergy is generated by an oxidation reaction and a reduction reaction.

Currently, a carbon-based material is mainly used as an electrode activematerial constituting a negative electrode of a lithium secondarybattery. In the case of graphite, the theoretical capacity is about 372mAh/g, and the actual capacity of the currently commercialized graphiteis realized to an extent of about 350 to 360 mAh/g. However, thecapacity of a carbon-based material such as graphite is not compatiblewith a lithium secondary battery requiring a high-capacity negativeelectrode active material.

In order to meet such a demand, there is an example in which asilicon-based material is used as a negative electrode active material,the silicon-based material which exhibits a higher charge/dischargecapacity than a carbon-based material, and which is a metal that iscapable of being electrochemically alloyed with lithium. However, such asilicon-based material has a high capacity, but has a very highelectrode expansion rate compared with that of a carbon-based material,and has very low charge/discharge efficiency, so that there is a problemin that it is difficult to use a silicon-based material in a largeproportion in a negative electrode. In addition, due to thecharge/discharge characteristic thereof, charging at a low voltage lastsfor a very long time so that the entire charge/discharge time of asecondary battery is significantly delayed.

RELATED ART DOCUMENT

Korea Patent Publication No. 2014-0117947

DISCLOSURE OF THE INVENTION Technical Problem

The present invention relates to a negative electrode for a secondarybattery, and a method for producing the same, and more particularly, toa negative electrode for a secondary battery for suppressing the volumeexpansion of a negative active material due to the intercalation anddeintercalation of lithium, and a method for producing the same.

Technical Solution

A negative electrode for a secondary battery according to an embodimentof the present invention includes a carbon-based active material; aconductive material; a silicon-based active material-polymer bindercombination including a silicon-based active material, and a polymerbinder for suppressing the expansion of the silicon-based activematerial bonded to a particle surface of the silicon-based activematerial; and a water-based binder.

The silicon-based active material may be included in an amount of 5 to25 wt % based on the total weight of active material including thecarbon-based active material and the silicon-based active material.

The polymer binder may be included in an amount of 25 to 45 wt % basedon the weight of the silicon-based active material.

The polymer binder may include a hydrophilic polymer material having ahydroxy group (—OH).

The polymer binder may include at least one of polyacrylic acid (PAA),polyvinyl alcohol (PVA), or sodium-polyacrylate (Na-PA).

The water-based binder may be included in an amount of 30 to 40 wt %based on the weight of the polymer binder.

The water-based binder may include a rubber-based material.

The water-based binder may include at least one of styrene butadienerubber (SBR), acrylonitrile butadiene rubber, acrylic rubber, butylrubber, or fluoro rubber.

The negative electrode for a secondary battery according to anembodiment of the present invention may have a discharge capacityretention rate of 90% or greater up to the fifth cycle under theconditions of charging at 0.5 C and discharging at 0.5 C.

In addition, a method for producing a negative electrode for a secondarybattery according to an embodiment of the present invention includespreparing a carbon-based active material; preparing a pre-dispersionslurry by mixing a silicon-based active material, a conductive material,and a polymer binder in a dispersion medium; mixing the carbon-basedactive material to the pre-dispersion slurry; preparing a negativeelectrode slurry by mixing a water-based binder with the mixture inwhich the carbon-based active material is mixed; applying the preparednegative electrode slurry on a current collector; and forming a negativeelectrode by removing moisture from the negative electrode slurry.

The pre-dispersion slurry may be formed by dispersing a silicon-basedactive material-polymer binder combination in which the silicon-basedactive material and a polymer binder are bonded, and the conductivematerial.

The solid content of the pre-dispersion slurry may be controlled by theviscosity of the pre-dispersion slurry.

The solid content of the pre-dispersion slurry may be controlled suchthat the pre-dispersion slurry has a viscosity of 3000 to 10000 cp.

The method for producing a negative electrode for a secondary batteryaccording to an embodiment of the present invention may further include,after mixing the carbon-based active material with the pre-dispersionslurry, mixing a thickener with the mixture in which the carbon-basedactive material is mixed.

The negative electrode slurry may be controlled to have a solid contentof 40 to 50 wt % based on the total weight of the negative electrodeslurry by controlling the amount of the thickener to be mixed.

Advantageous Effects

According to the negative electrode for a secondary battery and themethod for producing the same according to an embodiment of the presentinvention, by using a silicon-based active material-polymer bindercombination in which a polymer binder for suppressing the expansion of asilicon-based active material is adsorbed to the silicon-based activematerial, it is possible to effectively suppress the volume expansion ofthe silicon-based active material.

Also, in the case in which a negative electrode active materialincluding both a carbon-based active material and a silicon-based activematerial is used, a pre-dispersion slurry is prepared by pre-dispersingthe silicon-based active material in a polymer binder such that thepolymer binder may be selectively adsorbed on the surface of thesilicon-based active material with strong adhesion strength.

In addition, a polymer binder having rigidity may be softened with awater-based binder having high wettability to an electrolyte so that itis possible to prevent cracking or breakage of an electrode. Also,adhesion strength to a current collector may be improved, therebyimproving durability, and the lifespan characteristic of anelectrochemical device manufactured by using the same may be remarkablyimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the change in charge/discharge efficiencyaccording to the content of a polymer binder.

FIG. 2 is a diagram showing the change in charge/discharge efficiencyaccording to the content of a conductive material.

FIG. 3 is a diagram showing the change in viscosity according to thesolid content of a pre-dispersion slurry.

FIG. 4 is a diagram showing the change in sedimentation height accordingto the solid content of a pre-dispersion slurry.

FIG. 5 is a diagram showing the result of comparative analysis of theadhesion strength of a secondary battery using a negative electrodeaccording to an embodiment of the present invention.

FIG. 6 is a diagram showing the discharge rate of a secondary batteryusing a negative electrode according to an embodiment of the presentinvention.

FIG. 7 is a diagram showing the charge/discharge result of a secondarybattery using a negative electrode according to an embodiment of thepresent invention.

FIG. 8 is a diagram showing the change in electrode thickness of asecondary battery using a negative electrode according to an embodimentof the present invention.

MODE FOR CARRYING OUT THE INVENTION

A negative electrode for a secondary battery, and a method for producingthe same according to the present invention provide a technical featurecapable of suppressing the volume expansion of a negative electrodeactive material due to the intercalation and deintercalation of lithium.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention should not be construed as limited to the embodiments setforth herein, but may be embodied in various forms different from eachother. Rather, these embodiments are provided so that the disclosure ofthe present invention will be thorough and complete, and will fullyconvey the scope of the present invention to those skilled in the art.Like reference numerals refer to like elements throughout.

A negative electrode for a secondary battery according to an embodimentof the present invention includes a carbon-based active material; aconductive material; a silicon-based active material-polymer bindercombination including a silicon-based active material, and a polymerbinder for suppressing the expansion of the silicon-based activematerial bonded to a particle surface of the silicon-based activematerial; a thickener; and a water-based binder.

A conventional negative electrode for a secondary battery increases theloading of a negative electrode active material formed on a currentcollector, increases the number of stacks of an electrode in amanufacturing process, or increases the number of winding of anelectrode to achieve a high capacity. However, such a method has alimitation in application due to the structure of an electrode.

In order to solve this problem, there is provided a method in which acertain amount of a negative electrode active material is substitutedwith a silicon-based material which is a high-capacity material.However, in the case in which a silicon-based active material isincluded to be substituted with a certain amount of a negative electrodeactive material, cracks are generated and pulverized due to a largevolume change accompanied by charging and discharging of lithium, and asecondary battery using such a negative electrode active material stillhas problems in that the capacity thereof is rapidly reduced ascharge/discharge cycles progresses, and the cycle lifespan is shortened.

According to an embodiment of the present invention, when substituting acertain amount of a negative electrode active material with asilicon-based material which is a high-capacity material, asilicon-based active material-polymer binder combination in which asilicon-based active material, and a polymer binder for suppressing theexpansion of the silicon-based active material are bonded is used. Sucha silicon-based active material-polymer binder combination may be formedfrom a pre-dispersion aqueous solution prepared by pre-dispersing ahydrophilic polymer which is adsorbed on a particle surface of thesilicon-based active material but has very low wettability to anelectrolyte with a conductive material and the silicon-based activematerial.

In addition, according to an embodiment of the present invention, sincethe polymer binder adsorbed on a particle surface of the silicon-basedactive material has rigidity, in order to prevent the cracking orbreakage of an electrode and to improve adhesion strength to a currentcollector, the polymer binder is softened with a water-based binderhaving high wettability to an electrolyte. Such a method for producing anegative electrode from a pre-dispersion aqueous solution will bedescribed later with reference to a method for producing a negativeelectrode for a secondary battery according to an embodiment of thepresent invention.

The carbon-based active material is not particularly limited as long asit is capable of intercalating and deintercalating lithium. For example,the carbon-based active material may be one material or a mixture of twoor more materials selected from the group consisting of graphite,graphitizable carbon (also referred to as soft carbon),non-graphitizable carbon (also referred to as hard carbon), carbonblack, grapheme, and a graphene oxide.

The silicon-based active material is used to substitute a certain amountof carbon-based active material, and may be one material or a mixture oftwo or more materials selected from the group consisting of Si, SiOx,and an Si alloy. The silicon-based active material is substituted with acertain amount of carbon-based active material in a range that increasesthe capacity of a negative electrode, and at the same time, prevents theexcessive expansion thereof, and may be included in an amount of 5 to 25wt % based on the total weight of active material including thecarbon-based active material and the silicon-based active material.

The polymer binder is adsorbed on at least a portion of the surface ofthe silicon-based active material to be used in order to suppress theexpansion of the silicon-based active material. The polymer binder isadsorbed on a particle surface of the silicon-based active material, butmay include a hydrophilic polymer having very low wettability to anelectrolyte, and the hydrophilic polymer has a plurality of hydroxylgroups (—OH) capable of hydrogen bonding which is a strong bonding to bedispersed in water. As a result, the polymer binder has rigidity, has ahigh possibility of being adsorbed on the surface of the silicon-basedactive material due to strong bonding force, and has a characteristic ofnot being wet in an electrolyte which is an organic solution due tostrong hydrophilicity.

As described above, the polymer binder includes a polymer materialhaving strong hydrophilicity. For example, the polymer binder mayinclude one material or a mixture of two or more materials selected fromthe group consisting of a polyethylene oxide, a polypropylene oxide,polyacrylic acid, polyvinyl alcohol, polyvinyl acetate, andsodium-polyacrylate. Preferably, the polymer binder may include at leastone of polyacrylic acid (PAA), polyvinyl alcohol (PVA), orsodium-polyacrylate (Na-PA), all of which have excellent adsorptionforce to a silicon-based active material.

The polymer binder may be included in a range that prevents thedisconnection of a conductive network and maximizes the charge/dischargeefficiency of a secondary battery, and when used in an amount of 25 wt %or greater based on the total weight of the silicon-based activematerial, the charge/discharge efficiency of a secondary battery may berapidly increased and maintained. However, if the content of the polymerbinder is excessively increased, the viscosity of a pre-dispersionslurry increases so that the dispersibility is deteriorated and theworkability is reduced in a manufacturing process of a negativeelectrode. Therefore, the content may be controlled to be 45 wt % orless based on the total weight of the silicon-based active material.

FIG. 1 is a diagram showing the change in charge/discharge efficiencyaccording to the content of a polymer binder.

In FIG. 1, the silicon-based active material described above was used,and after fixing the weight percentage of the conductive material at 1wt % based on the total weight of the pre-dispersion slurry, thecharge/discharge efficiency according to charging and discharging wasconfirmed while increasing the content of the polymer binder. Here, thecharge/discharge efficiency is a result confirmed in the first cycle andthe second cycle. The reason for confirming the initial charge/dischargeefficiency is that due to the characteristics of the silicon-basedmaterial which a high-capacity material, the disconnection of theconductive network caused by the volume expansion during the initialcharging has the greatest influence on the reduction of thecharge/discharge efficiency, and the use of the polymer binder maysuppress this.

As shown in FIG. 1, in the case in which the ratio of the weightpercentage of the polymer binder to the weight percentage of thesilicon-based active material is 25.3% or greater, it can be seen thatthe charge/discharge efficiency is increased, and especially in thesecond cycle, it can be seen that the charge/discharge efficiency israpidly increased. In the case in which the content of the polymerbinder is further increased, it can be seen that the charge/dischargeefficiency is maintained at a similar level. Therefore, in using thesilicon-based active material, when the content of the polymer binder is25 wt % or greater based on the total weight of the silicon-based activematerial, it is possible to improve the charge/discharge efficiency.

The conductive material is not particularly limited as long as it hasconductivity without causing a side reaction with other elements of asecondary battery. For example, the conductive material may include onematerial or a mixture of two or more materials selected from the groupconsisting of graphite such as natural graphite, artificial graphite,and the like; carbon black such as carbon black (super-p), acetyleneblack, ketjen black, channel black, furnace black, lamp black, summerblack, and the like; a conductive fiber such as a carbon fiber, a carbonnanofiber, a metal fiber, and the like; metal powder such as carbonfluoride, aluminum, nickel powder, and the like; a conductive whiskersuch as a zinc oxide, potassium titanate, and the like; a conductivemetal oxide such as a titanium oxide, and the like; and a conductivematerial such as a polyphenylene derivative, and the like.

The conductive material may be included in a range such that a negativeelectrode for a secondary battery maintains conductivity. When used inan amount of 5 wt % or greater based on the total weight of thesilicon-based active material, the charge/discharge efficiency israpidly increased, and when used in an amount of 10 wt % or greater, itis possible to obtain a charge/discharge efficiency of 95% or more. Asthe content of the conductive material is increased, it is possible toform an effective conductive network. However, in order to control thesolid concentration of the generated negative electrode slurry, thecontent may be controlled to be 20 wt % or less based on the totalweight of the silicon-based active material.

FIG. 2 is a diagram showing the change in charge/discharge efficiencyaccording to the content of a conductive material.

In FIG. 2, the silicon-based active material described above was used,and after fixing the ratio of the weight percentage of the silicon-basedactive material to the weight percentage of the polymer binder at 25%,the charge/discharge efficiency according to charging and dischargingwas confirmed while increasing the content of the polymer binder. Here,the charge/discharge efficiency is a result confirmed in the first cycleand the second cycle as in FIG. 1.

As show in in FIG. 2, in the case in which the ratio of the weightpercentage of the conductive material to the weight percentage of thesilicon-based active material is 5.3% or greater, it can be seen thatthe charge/discharge efficiency is rapidly increased. As the content ofthe conductive material was increased, the charge/discharge efficiencywas continuously increased. Accordingly, the conductive material may beincluded in an amount of 5 wt % or greater based on the weight of thesilicon-based active material.

In addition, when the charge/discharge efficiency of 95% or greater isdetermined to be the maximum efficiency of the silicon-based activematerial, the required content of the conductive material is 10 wt % orgreater based on the weight of the silicon-based active material, and inthe case of 15 wt % or greater, much better performance improvement maybe achieved.

The thickener may be selectively included to determine the solid contentof a negative electrode slurry for manufacturing a negative electrode.That is, the thickener is added when needed in order to control thesolid content of the negative electrode slurry to be in an appropriaterange, and may be one material or a mixture of two or more materialsselected from cellulose-based materials such as carboxy methyl cellulose(CMC), methyl cellulose, hydroxypropyl cellulose, and the like. Thecontent of the thickener is determined according to the solid content ofthe negative electrode slurry, and may be included in an amount of 0 to5 wt % based on the total weight of the solid content in the negativeelectrode slurry so that the negative electrode slurry has a solidcontent of 40 to 50 wt % based on the total weight.

The water-based binder is added to assist the flexibility of anelectrode that is the negative electrode, and the adhesion strength ofthe electrode. The water-based binder may include a rubber-basedmaterial, and may be one material or a mixture of two or more materialsselected from the group consisting of styrene butadiene rubber (SBR),acrylonitrile butadiene rubber, acrylic rubber, butyl rubber, and fluororubber.

The polymer binder is characterized by having a plurality of hydroxygroups (—OH) capable of hydrogen bonding which is generally strongbonding, in order to be dispersed in water. As a result, the polymerbinder has very stiff rigidity, has a high possibility of being adsorbedon a particle surface of the silicon-based active material with strongadhesion strength, and has very strong hydrophilicity, thereby having acharacteristic of not being wet in an electrolyte which is an organicsolution.

Here, the polymer binder has strong adhesion strength, and therefore,the rigidity of the polymer binder itself is very high so that aphenomenon of cracking or breakage of the electrode may occur. Thus, itis necessary to add the water-based binder.

The water-based binder may be included in an amount of 30 to 40 wt %based on the weight of the polymer binder. When included in an amount of30 wt % or less, problems such as lack of adhesion strength and crackingof the electrode may occur.

When included in an amount of 40 wt % or greater, the water-based binderhaving good wettability to an electrolyte may become wet in theelectrolyte, thereby reducing the rigidity of the polymer binderexcessively so that the effect of suppressing the volume expansion ofthe silicon-based active material is deteriorated.

As described above, the silicon-based active material-polymer bindercombination is formed from a pre-dispersion slurry prepared bypre-dispersing a hydrophilic polymer adsorbed to a particle surface ofthe silicon-based active material but having very low wettability to anelectrolyte with the conductive material and the silicon-based activematerial. That is, in order to use the silicon-based active materialhaving high charge/discharge capacity, it is very important to suppressthe volume expansion of the silicon-based active material due tocharging and discharging, and the volume expansion of the silicon-basedactive material is suppressed by the polymer binder adsorbed to thesilicon-based active material.

The pre-dispersion slurry is then mixed with the carbon-based activematerial, and the water-based binder is added thereto to prepare thenegative electrode slurry. In preparing a negative electrode using anegative electrode active material including the carbon-based activematerial and the silicon-based active material, by preparing apre-dispersion slurry by pre-dispersing only the silicon-based activematerial with the conductive material and the polymer binder, it ispossible to effectively suppress the volume expansion of thesilicon-based active material.

That is, in the case of preparing the negative electrode slurry bydispersing the conductive material and the polymer binder with thenegative electrode active material including the carbon-based activematerial and the silicon-based active material, when mixing the polymerbinder, the object on which the polymer binder is adsorbed is notlimited to the silicon-based active material, but is extended to thecarbon-based active material. Accordingly, the degree to which thepolymer binder is adsorbed to the silicon-based active material isreduced so that the volume expansion of the silicon-based activematerial may not be sufficiently suppressed.

On the other hand, when the pre-dispersion slurry is prepared bypre-dispersing the silicon-based active material with the conductivematerial and the polymer binder, it is possible that the polymer binderis selectively adsorbed on the surface of the silicon-based activematerial. That is, the surface of the silicon-based active materialgenerally has hydrophilicity, and the polymer binder to be mixedincludes a hydrophilic polymer material. Therefore, it is possible toform the silicon-based active material-polymer binder combination byeffectively attaching the polymer binder to the surface of thesilicon-based active material with strong hydrogen bonding force alongwith van der Walls force.

Hereinafter, a method for producing a negative electrode for a secondarybattery according to an embodiment of the present invention will bedescribed in detail.

The method for producing a negative electrode for a secondary batteryaccording to an embodiment of the present invention includes preparing acarbon-based active material S100; preparing a pre-dispersion slurry bymixing a silicon-based active material, a conductive material, and apolymer binder in a dispersion medium S200; mixing the carbon-basedactive material with the pre-dispersion slurry S300; preparing anegative electrode slurry by mixing a water-based binder with themixture in which the carbon-based active material is mixed S500;applying the prepared negative electrode slurry on a current collectorS600; and forming a negative electrode by removing moisture from thenegative electrode slurry S700.

Here, the preparing a carbon-based active material S100 and thepreparing a pre-dispersion slurry by mixing a silicon-based activematerial, a conductive material, and a polymer binder in a dispersionmedium S200 are not in a time-series sequencing relation to each other.The preparing a pre-dispersion slurry S200 may be performed after thepreparing a carbon-based active material S100 is performed, or thepreparing a carbon-based active material S100 may be performed after thepreparing a pre-dispersion slurry S200 is performed. The preparing acarbon-based active material S100 and the preparing a pre-dispersionslurry S200 may be performed simultaneously.

In the preparing a carbon-based active material S100, a carbon-basedactive material in a power state is first prepared. Here, as thecarbon-based active material, various carbon-based materials capable ofintercalating and deintercalating lithium may be used as describedabove.

In the preparing a per-dispersion slurry S200, a pre-dispersion mixtureis first formed by mixing a silicon-based active material in a powderstate, a conductive material, and a polymer binder. Then, a dispersionmedium, for example, water, is added to the mixture and stirred toprepare a pre-dispersion slurry. The adding of water to the mixture andstirring may be carried out by a saw blade type mixer having a rotationspeed exceeding 2000 to 2500 rpm such as a homogenizer, a universalstirrer, a clear mixer, and a fill mixer which are known in the art, orby equipment such as a bead mill, a ball mill, a basket mill, anattrition mill, and the like, of which a filling material such as beadsare filled thereto to perform mixing.

In the case of forming a per-dispersion mixture by mixing asilicon-based active material, a conductive material, and a polymerbinder, the per-dispersion slurry is formed by dispersing thesilicon-based active material-polymer binder combination in which thesilicon-based active material and the polymer binder is bonded and theconductive material in water which is a dispersion medium.

Here, the solid content of the pre-dispersion slurry is controlled bythe viscosity of the pre-dispersion slurry.

In detail, the viscosity of the pre-dispersion slurry prepared by thesame mixing process is determined by the ratio of the polymer binder tothe silicon-based active material. That is, as the content of thepolymer binder is increased, the viscosity of the pre- dispersion slurryis increased, and in the case of the silicon-based active material, thesolid density thereof is from 2.1 to 2.35 g/cm³. Accordingly, in orderto suppress the sedimentation of the silicon-based active material afterthe per-dispersion slurry is prepared, a viscosity of a certain range isrequired.

Here, the range of the viscosity is limited in order to improve theworkability while suppressing the sedimentation of the silicon-basedactive material to the maximum. The pre-dispersion slurry may have aviscosity of 3000 to 10000 cp. That is, in the case in which theviscosity of the pre-dispersion slurry is less than 3000 cp, thesediment rate of the silicon-based active material is increased, so thatparticles of the silicon-based active material sink rapidly during therest period of the pre-dispersion slurry, resulting in the deteriorationof the physical properties of the pre-dispersion slurry. In the case inwhich the viscosity of the pre-dispersion slurry is greater than 10000cp, the dispersibility of the pre-dispersion slurry is reduced and theworkability in performing the process becomes very low.

FIG. 3 is a diagram showing the change in viscosity according to thesolid content of a pre-dispersion slurry. FIG. 4 is a diagram showingthe change in sedimentation height according to the solid content of apre-dispersion slurry. In FIGS. 3 and 4, the case of which the ratio ofthe polymer binder to the silicon-based active material is 1:0.24 willbe described as an example.

As shown in FIG. 3, in the case in which the ratio of the polymer binderto the silicon-based active material is 1:0.24, when the solid contenthas a value of 22 to 25 wt % based on the total weight of thepre-dispersion slurry, the pre-dispersion slurry has a viscosity of 3000to 10000 cp. Therefore, in the case in which the ratio of the polymerbinder to the silicon-based active material is 1:0.24, the suitablesolid content of the pre-dispersion slurry is determined to be a valueof 22 to 25 wt %.

In FIG. 4, after the pre-dispersion slurry is prepared with the ratio ofthe polymer binder to the silicon-based active material of 1:0.24, acertain amount thereof is collected in a vial capable of measuringlength, and the height of the bottom surface of the pre-dispersionslurry is measured. Thereafter, post-processing is performed for 24hours at room temperature, and then a bar having a certain length isinserted into the vial to measure the height of the bottom surface ofthe pre-dispersion slurry again.

In the case of the pre-dispersion slurry, the sedimentation rate isdetermined according to a certain viscosity ratio, and the sedimentedsilicon-based active material and the polymer binder are accumulated onthe bottom surface. From the height measured by the inserted bar after24 hours, it is possible to measure the height of the sedimentationlayer finally sedimented on the bottom of the vial, and to confirmproportionally how much has been sedimented during a certain period oftime. Here, the reason for measuring the height of the sedimented layerafter 24 hours is that when mass producing negative electrodes for asecondary battery, a rest period is given according to a process, andthe rest period is usually 12 to 24 hours. Therefore, the height of thesedimentation layer was measured after 24 hours.

As shown in FIG. 4, in the case in which the ratio of the polymer binderto the silicon-based active material is 1:0.24, when the solid contentof the pre-dispersion slurry has a value of 22 to 25 wt % based on thetotal weight of the pre-dispersion slurry, it can be seen that the ratioof the sedimentation layer is within 3%. From this, it can be seen thatin the case in which the solid content of the pre-dispersion slurry is22 wt % or greater, the pre-dispersion slurry has a viscosity of 3000 to10000 cp, so that it is possible to effectively achieve thesedimentation of the silicon-based active material and the polymerbinder.

Therefore, in the preparing a pre-dispersion slurry S200, the solidcontent of the pre-dispersion slurry may be controlled such that thepre-dispersion slurry may have a viscosity of 3000 to 10000 cp. Asdescribed above, since the polymer binder may be included in an amountof 25 to 45 wt % based on the weight of the silicon-based activematerial, in order to have the viscosity of the pre-dispersion slurrywithin a range satisfying the above range, the pre-dispersion slurry mayhave a solid content of 15 to 25 wt % based on the total weight.

In the mixing the carbon-based active material S300, when thepre-dispersion slurry is prepared by the process described above, anegative electrode slurry is prepared by mixing the pre-dispersionslurry with the carbon-based active material. By preparing thepre-dispersion slurry as described above, and mixing the carbon-basedactive material with the pre-dispersion slurry, the polymer binder maybe selectively adsorbed on the surface of the silicon-based activematerial. That is, the surface of the silicon-based active materialgenerally has hydrophilicity, and the polymer binder to be mixedincludes a hydrophilic polymer material, thereby being effectivelyattached to the surface of the silicon-based active material with stronghydrogen bonding force along with van der Walls force, so that it ispossible to form a silicon-based active material-polymer bindercombination.

Thereafter, the method for producing a negative electrode for asecondary battery may further include mixing a thickener with themixture in which the carbon-based active material is mixed S400. Thethickener is selectively included in the mixture to determine the solidcontent of a negative electrode slurry for manufacturing a negativeelectrode, and the content thereof is controlled to be in an appropriaterange in order to control the solid content of the negative electrodeslurry to be prepared afterwards. In the case in which the negativeelectrode slurry has a solid content of 15 to 25 wt % based on the totalweight, it is possible to improve the workability while applying, and atthe same time, to improve the rate of drying for removing water from thenegative electrode slurry. Therefore, the thickener may be included inan amount of 0 to 5 wt % based on the total weight of the solid contentin the negative electrode slurry such that the negative electrode slurryhas the solid content described above.

In the preparing a negative electrode slurry S500, in the case in whichthe mixture in which the carbon-based active material is mixed, or thethickener is included in the pre-dispersion slurry, the negativeelectrode slurry is prepared by mixing a water-based binder with themixture in which the carbon-based active material and the thickener aremixed with the pre-dispersion slurry.

As described above, the polymer binder has very stiff rigidity, has ahigh possibility of being adsorbed on a particle surface of thesilicon-based active material with strong adhesion strength, and hasvery strong hydrophilicity, thereby having a characteristic of not beingwet in an electrolyte which is an organic solution. By the addition ofthe water-based binder, the polymer binder having rigidity is softenedwith the water-based binder having high wettability to an electrolyte sothat it is possible to prevent cracking or breakage of the electrode,and to improve the adhesion strength to the current collector.

Here, the negative electrode slurry may have a solid content of 40 to 50wt % based on the total weight. That is, the pre-dispersion slurry has asolid content of 15 to 25 wt % based on the total weight of thepre-dispersion slurry, and by mixing the carbon-based active materialand the water-based binder, or by further mixing the thickener therewithwhen needed, the negative electrode slurry may have a solid content of40 to 50 wt % based on the total weight of the negative electrodeslurry. By maintaining the solid content of the negative electrodeslurry to be in the range above described, it is possible to improve theworkability while applying, and at the same time, to improve the rate ofdrying for removing water from the negative electrode slurry.

In the applying the negative electrode slurry on a current collectorS600, the negative electrode slurry prepared in the preparing a negativeelectrode S500 is applied on the current collector. Here, in general, acurrent collector having a thickness of 3 μm to 500 μm may be used, andis not particularly limited as long as it has high conductivity withoutcausing chemical change in a secondary battery. For example, copper,stainless steel, aluminum, nickel, titanium, sintered carbon, oraluminum, or stainless steel, the surface of which are treated withcarbon, nickel, titanium, or silver, and the like may be used.

The forming a negative electrode S700 is performed by removing waterfrom the negative electrode slurry. The water included in the negativeelectrode slurry may be removed by drying the negative electrode slurryapplied on the current collector, and a common method used for forming anegative electrode may be used.

Hereinafter, the result according to an embodiment of the presentinvention will be described in comparison with the experimental resultsof each comparative example and example. Here, the following examplesare provided to illustrate the present invention, and the scope of thepresent invention is not limited by experimental conditions.

COMPARATIVE EXAMPLE 1

As a negative electrode active material, a carbon-based active materialand a silicon-based active material were used in an amount of 90 wt %and 10 wt % respectively based on the total weight of the negativeelectrode active material.

A conductive material of carbon black was dispersed in water which is adispersion medium, and the negative electrode active material was mixedthereto as a whole. Then, carboxymethyl cellulose (CMC) which is athickener, and styrene butylene rubber (SBR) which is a binder weresequentially mixed therewith to prepare a negative electrode slurry, andthe negative electrode slurry was applied on a current collector anddried to form a negative electrode for a secondary battery.

The negative electrode for a secondary battery was formed of thenegative electrode active material of 95.6 wt %, the conductive materialof 1.0 wt %, the thickener of 1.1 wt %, and the binder of 2.3 wt %.

Here, the loading of the negative electrode for a secondary battery ofComparative Example 1 was 204 mg/25 cm², and the porosity thereof was33.9%.

EXAMPLE 1

As a negative electrode active material, a carbon-based active materialand a silicon-based active material were used in an amount of 90 wt %and 10 wt % respectively based on the total weight of the negativeelectrode active material.

A conductive material of carbon black, a silicon-based active material,and a polymer binder were pre-dispersed in water which is a dispersionmedium to prepare a pre-dispersion slurry, and a carbon-based activematerial was mixed therewith. In addition, carboxymethyl cellulose (CMC)which is a thickener was mixed with a mixture in which the carbon-basedactive material was mixed, and styrene butylene rubber (SBR) which is abinder was sequentially mixed therewith to prepare a negative electrodeslurry. Thereafter, the negative electrode slurry was applied on acurrent collector and dried to form a negative electrode for a secondarybattery. Here, 9.5 wt % of the polymer binder based on the total weightof the polymer binder was first added to prepare a first pre-dispersionslurry, and then the prepared first pre-dispersion slurry and theremaining 90.5 wt % of the polymer binder were further mixed to preparea second pre-dispersion slurry. The thickener and the water-based binderwere sequentially mixed with the second pre-dispersion slurry to preparethe negative electrode slurry.

The negative electrode for a secondary battery was formed of thenegative electrode active material of 95.6 wt %, the polymer binder of2.4 wt %, the conductive material of 1.0 wt %, the thickener of 0.15 wt%, and the water-based binder of 0.85 wt %.

Here, the loading of the negative electrode for a secondary battery ofExample 2 was 205 mg/25 cm², and the porosity thereof was 35.4%.

EXAMPLE 2

As a negative electrode active material, a carbon-based active materialand a silicon-based active material were used in an amount of 90 wt %and 10 wt % respectively based on the total weight of the negativeelectrode active material.

A conductive material of carbon black, a silicon-based active material,and a polymer binder were pre-dispersed in water which is a dispersionmedium to prepare a pre-dispersion slurry, and a carbon-based activematerial was mixed therewith. In addition, carboxymethyl cellulose (CMC)which is a thickener was mixed with a mixture in which the carbon-basedactive material was mixed, and styrene butylene rubber (SBR) which is abinder was sequentially mixed therewith to prepare a negative electrodeslurry. Thereafter, the negative electrode slurry was applied on acurrent collector and dried to form a negative electrode for a secondarybattery. In this case, the polymer binder as a whole was mixed with thesilicon-based active material and the conductive material to prepare thepre-dispersion slurry.

The negative electrode for a secondary battery was formed of thenegative electrode active material of 95.6 wt %, the polymer binder of2.4 wt %, the conductive material of 1.0 wt %, the thickener of 0.15 wt%, and the water-based binder of 0.85 wt %.

Here, the loading of the negative electrode for a secondary battery ofExample 2 was 205 mg/25 cm², and the porosity thereof was 35.4%.

FIG. 5 is a diagram showing the result of comparative analysis of theadhesion strength of a secondary battery using a negative electrodeaccording to an embodiment of the present invention.

As shown in FIG. 5, in the case of Comparative Example 1, the adhesionstrength to the current collector was higher than that of Example 1 andExample 2. However, the difference is insignificant. Comparative Example1, Example 1, and Example 2 were all confirmed to have a certain levelof adhesion strength.

FIG. 6 is a diagram showing the discharge rate of a secondary batteryusing a negative electrode according to an embodiment of the presentinvention.

In FIG. 6, discharge rates of 0.5 C, 1.0 C, 2.0 C, 3.0 C, and 4.0 C wereconfirmed based on the discharge capacity of 0.1 C. As shown in FIG. 6,in the cases of Example 1 and Example 2, the discharge ratecharacteristic thereof were very high when compared to that ofComparative Example 1. This is the result of the polymer binder beingadsorbed on the surface of the silicon-based active material, and it canbe seen that the polymer binder effectively suppresses the volumeexpansion due to the charge/discharge of the silicon-based activematerial by using strong adhesion strength thereof.

FIG. 7 is a diagram showing the charge/discharge result of a secondarybattery using a negative electrode according to an embodiment of thepresent invention.

As shown in FIG. 7, when the lifespan evaluation is conducted byperforming charging and discharging at 0.5 C/0.5 C for 30 cycles, it canbe seen that Comparative Example 1 has a low discharge capacityretention rate because the volume expansion was not at all suppressed.In comparison, when a polymer binder is used, there is a superiority inlifespan characteristic. In particular, both Example 1 and Example 2have a discharge capacity retention rate of 90% or greater up to thefifth cycle, and therefore, it can be confirmed that the volumeexpansion of the silicon-based active material is effectively suppressedby the polymer binder.

FIG. 8 is a diagram showing the change in electrode thickness of asecondary battery using a negative electrode according to an embodimentof the present invention.

As shown in FIG. 8, in the case of Comparative Example 1, the fullcharge thickness change ratio of the electrode was measured to be 43.9%.On the other hand, in the case of Example 1, the full charge thicknesschange ratio of the electrode was measured to be 48.4%, and in the caseof Example 2, the full charge thickness change ratio of the electrodewas measured to be 41.4%.

Here, when the full charge thickness change ratio was confirmed after 30cycle evaluation, Comparative Example 1 showed a thickness expansionrate lower than that of Example 1. However, in the case of ComparativeExample 1, since the result of lifespan evaluation described above showsa very low lifespan, it cannot be seen that the performance of thesilicon-based active material is exhibited. On the other hand, it can beconfirmed that Example 2 shows a thickness expansion rate lower thanthat of Comparative Example 1.

From the results, it can be seen that the lifespan characteristic of asecondary battery was improved both in Example 1 and Example 2 whencompared to Comparative Example 1, and in preparing the pre-dispersionslurry, in the case of Example 1 in which the final pre-dispersionslurry is prepared by the first pre-dispersion slurry and the secondpre-dispersion slurry, it is possible to improve the adhesion strengthto the current collector. In the case of Example 2 in which thepre-dispersion slurry is prepared by mixing the polymer binder as awhole with the silicon-based active material and the conductivematerial, it is possible to effectively reduce the thickness expansionrate.

That is, according to the negative electrode for a secondary battery andthe method for producing the same according to an embodiment of thepresent invention, by using a silicon-based active material-polymerbinder combination in which a polymer binder for suppressing theexpansion of a silicon-based active material is adsorbed to thesilicon-based active material, it is possible to effectively suppressthe volume expansion of the silicon-based active material.

Also, in the case in which a negative electrode active materialincluding both a carbon-based active material and a silicon-based activematerial is used, a pre-dispersion slurry is prepared by pre-dispersingthe silicon-based active material in a polymer binder such that thepolymer binder may be selectively adsorbed on the surface of thesilicon-based active material with strong bonding force.

In addition, a polymer binder having rigidity may be softened with awater-based binder having high wettability to an electrolyte so that itis possible to prevent cracking or breakage of an electrode. Also,adhesion strength to a current collector may be improved, therebyimproving durability, and the lifespan characteristic of anelectrochemical device manufactured by using the same may be remarkablyimproved.

In the above, while the preferred embodiments of the present inventionhave been described and illustrated using specific terms, such terms areused only for the purpose of clarifying the invention. It is to beunderstood that various changes and modifications may be made to theembodiments of the present invention and the described terms withoutdeparting from the spirit and scope of the following claims. Suchmodified embodiments should not be individually understood from thespirit and scope of the present invention, but should be regarded asbeing within the scope of the claims of the present invention.

1. A negative electrode for a secondary battery comprising: acarbon-based active material; a conductive material; a silicon-basedactive material-polymer binder combination comprising a silicon-basedactive material, and a polymer binder bonded to a surface of thesilicon-based active material, wherein the polymer binder suppressesexpansion of the silicon-based active material; and a water-basedbinder.
 2. The negative electrode for a secondary battery of claim 1,wherein the silicon-based active material is included in an amount of 5to 25 wt % based on a total weight of active material including thecarbon-based active material and the silicon-based active material. 3.The negative electrode for a secondary battery of claim 1, wherein thepolymer binder is included in an amount of 25 to 45 wt % based on aweight of the silicon-based active material.
 4. The negative electrodefor a secondary battery of claim 1, wherein the polymer binder comprisesa hydrophilic polymer material having a hydroxy group (—OH).
 5. Thenegative electrode for a secondary battery of claim 1, wherein thepolymer binder comprises at least one of polyacrylic acid (PAA),polyvinyl alcohol (PVA), or sodium-polyacrylate (Na-PA).
 6. The negativeelectrode for a secondary battery of claim 1, wherein the water-basedbinder is included in an amount of 30 to 40 wt % based on a weight ofthe polymer binder.
 7. The negative electrode for a secondary battery ofclaim 1, wherein the water-based binder comprises a rubber-basedmaterial.
 8. The negative electrode for a secondary battery of claim 1,wherein the water-based binder comprises at least one of styrenebutadiene rubber (SBR), acrylonitrile butadiene rubber, acrylic rubber,butyl rubber, or fluoro rubber.
 9. A secondary battery comprising thenegative electrode of claim 1, said secondary battery having a dischargecapacity retention rate of 90% or greater up to a fifth cycle underconditions of charging at 0.5 C and discharging at 0.5 C.
 10. A methodfor manufacturing a negative electrode for a secondary battery,comprising: preparing a carbon-based active material; preparing apre-dispersion slurry by mixing a silicon-based active material, aconductive material, and a polymer binder in a dispersion medium; mixingthe carbon-based active material with the pre-dispersion slurry;preparing a negative electrode slurry by mixing a water-based binderwith the mixture in which the carbon-based active material is mixed withthe pre-dispersion slurry; applying the prepared negative electrodeslurry on a current collector; and fainting a negative electrode byremoving moisture from the negative electrode slurry.
 11. The method ofclaim 10, wherein the pre-dispersion slurry is formed by dispersing thesilicon-based active material-polymer binder combination in which thesilicon-based active material and the polymer binder are bonded, and theconductive material.
 12. The method of claim 10, wherein a solid contentof the pre-dispersion slurry is controlled by a viscosity of thepre-dispersion slurry.
 13. The method of claim 12, wherein the solidcontent of the pre-dispersion slurry is controlled such that thepre-dispersion slurry has a viscosity of 3000 to 10000 cp.
 14. Themethod of claim 10 further comprising, after mixing the carbon-basedactive material with the pre-dispersion slurry, mixing a thickener withthe mixture in which the carbon-based active material is mixed with thepre-dispersion slurry.
 15. The method of claim 14, wherein the negativeelectrode slurry is controlled to have a solid content of 40 to 50 wt %based on a total weight of the negative electrode slurry by controllingan amount of the thickener to be mixed.